Goal: To determine the three-dimensional structure of biological macromolecules (proteins, enzymes, and nucleic acids) using primarily the method of single-crystal x-ray diffraction. Major emphasis is given to macromolecular complexes and complex interactions between molecules. Two systems being studies include DNA replication and the energy-dependent destruction of damaged proteins. Efforts are also underway to determine the structure of retroviral proteins and enzymes. Importance: Knowledge of a macromolecule's three-dimensional structure is vital to understanding its biological function. Very often biological molecules can function only by arranging themselves into larger assemblies in the cell. The chosen model systems for DNA replication and energy-dependent proteolysis are processes indispensable to all living cells. Work to determine structures of retroviral target enzymes is essential for the structure-based drug development and other potential therapeutic strategies. Recent progress: We have determined the high resolution crystal structure of the RNase H (5' to 3' exonuclease) from bacteriophage T4, an enzyme vital to replication of the lagging DNA strand. An analysis of its active center revealed the binding of two magnesiums ions required for activity, and an insight into the structure and function of several enzymes from higher organisms involved in the repair of damaged DNA. Well-diffracting crystalline forms of a helicase accessory protein, CLP-P protease, retroviral integrase and envelope protein fragments have been obtained and structure characterizations are underway.